The suprising thing is that people are still ON some servers.
Not really. The authentication servers are down, not the individual servers people own. You're only authenticated when you first connect to a server, so people that are already on a server can continue playing. Anyone who disconnects or lags out on a server won't be able to reconnect.
If you add a big server like mc.hypixel.com and refresh it a few times you will see the number of people connected gradually falling as people disconnect and nobody is able to login.